Mortis was created by the Firstborn Nephilim as one of the Grand Abominations, weapons created for the purpose of committing world-spanning genocides against the enemies of their creators. Mortis was created as a shield that could absorb virtually any attack, including ones made by other small-scale Abominations. Once it absorbed an attack, it released ethereal energies that sought out and randomly killed someone who the attacker held dear, even if they were nowhere near Mortis at the time. After the defection of the Horsemen and the destruction of the Nephilim, Mortis was retrieved by Death during the Abomination Vault Crisis. He noted that the Abomination was half dead, and as such was at only a fraction of its former power. Nevertheless, it still maintained both the ability to block the unfailingly-lethal projectiles of the Abomination Black Mercy, and to project its counter-attack.

Mortis is a Buckler that combines Attack and Defense into one brutal weapon. Since ancient times Mortis has defended its wielder by responding to any attack with a surge of lethal energy that unerringly seeks out and punishes the opponent.— In-game Description, Darksiders 2

”

“

It’s a shield, but its purpose was never to protect the wearer—at least, not directly. When awakened and at full strength, Mortis reacted to any attack on the wearer by sending out a concentrated burst of profane energies. It was incredibly swift, capable of traversing an entire world in a matter of heartbeats.— Death to War, The Abomination Vault